This invention relates to an alkaline secondary battery, to a paste type positive electrode for alkaline secondary battery, and to a method for manufacturing an alkaline secondary battery.
The positive electrode for alkaline secondary battery can be generally classified into two types, i.e. a sintered type and a paste type. In view of enhancing the capacity of battery, a paste type positive electrode wherein a paste containing nickel hydroxide particles as an active material is filled in or coated on an electronic collector has been predominantly employed. It is imperative for this paste type positive electrode to ensure a sufficient electric contact between the particulate active material and the electronic collector so as to improve the utilization of the particulate active material. With a view to realize the aforementioned sufficient electric contact, various methods haven been adopted, e.g. a method wherein cobalt metal or a cobalt compound is added as a conductivity assistant to a paste and then converting the cobalt metal or compound into a high-order cobalt oxide of high conductivity by subjecting it to an initial charging, or a method wherein cobalt hydroxide is first formed on the surface of particulate nickel hydroxide, which is then subjected to a heat treatment in the presence of alkali so as to form a high order cobalt oxide of high conductivity on the surface of the particulate nickel hydroxide.
Although an alkali secondary battery provided with the aforementioned paste type positive electrode is effective in improving the utilization of the active material, the alkali secondary battery becomes poor in over-discharge property so that the lowering of capacity recovery ratio becomes prominent when the alkali secondary battery is stored for a long period of time or under a high temperature environment.
By the way, it is disclosed by Oshitani et al in J. Electrochem. Soc., 136 (1989) on page 1590 that a high order cobalt oxide formed as a conductive network on a paste type nickel electrode can be stably kept remained during the ordinary charging/discharging. Further, DENKI KAGAKU 63, No.1 (1995) describes on page 952 that if an alkali secondary battery provided with a paste type nickel positive electrode is left for a long period of time or under a high temperature environment, the battery voltage would be lowered and the capacity recovery of the battery at occasion of re-charging would be deteriorated.
Meanwhile, Japanese Patent Unexamined Publication Hei/5-314983 discloses a method of performing an initial charge of an alkali secondary battery wherein a calcium compound is added to the positive electrode and then the initial charging is performed in an atmosphere heated to 40 to 70.degree. C. This initial charging is intended to accelerate the dissolution of the calcium compound in the positive electrode thereby to obtain an alkali secondary battery exhibiting a stable initial performance.
On the other hand, Japanese Patent Unexamined Publication Hei/5-275082 discloses a method of manufacturing a nickel-hydrogen secondary battery which is provided with a nickel positive electrode and a negative electrode containing an AB.sub.2 -based hydrogen-absorbing alloy, wherein the initial charging is performed in an atmosphere heated up to 50 to 70.degree. C. so as to lower the charging efficiency of the positive electrode and to enhance the initial performance of the negative electrode.
However, it has been still difficult, in spite of this initial charging, to sufficiently enhance the capacity recovery ratio, once the battery is left for a long period of time or under a high temperature environment.